It seemed like an odd idea at the time, building tunnels under a busy road to help California tiger salamanders travel safely from their hillside homes to a nearby pond where the endangered species can breed.

The plan, however, appears to have worked, according to biologists who are studying the tunnels, built two years ago under Stony Point Road near Cotati.

"I haven't analyzed all the data. Everything is preliminary, but just a broad general conclusion is these crossing tunnels are working. They are functional, and salamanders are using them," said Tracy Bain, a graduate student at Sonoma State University.

The San Francisco resident is writing her thesis on the effectiveness of the tunnels for a masters degree in conservation biology.

"There are lots of things that fragment habitat of migrating animals," Bain said. "For these salamanders, who go from their upland habitat, where they are year-around, to the pond, the road is the problem."

The tunnels were constructed two years ago by Sonoma County, using a $150,000 grant from a Caltrans fund to offset environmental effects of roadwork.

It was an idea proposed four years ago by David Cook, a senior environmental specialist for the Sonoma County Water Agency who studies amphibians, such as frogs and salamanders.

California tiger salamanders are 8-inch amphibians with bright spots that live in gopher holes but come out during the first evening rains of winter, migrating as far as a half-mile to breed in ponds.

One such breeding area is near Cotati, where the tiger salamanders live in the uphill grasslands on one side of Stony Point Road. The breeding ponds, where they lay their eggs, is on the other side of the road.

"I was doing wildlife studies for a Water Agency proposed pipeline in the area and I found this major migration route that crosses Stony Point and a frequent mortality," Cook said.

The salamanders were listed as an endangered species in Sonoma County in 2003 by the U.S. Fish and Wildlife Service, becoming a lightning rod for criticism by developers and growth advocates, who complain the amphibians' protected status holds up projects.

Cook said, however, it is no different than protecting salmon, also on the endangered species list, and tunnels are no different than fish ladders.

Before the tunnels were built, the half-mile stretch of Stony Point Road was a "blood bath" as the salamanders were crushed by cars as they tried to cross the road, Cook said.

He said there could be several hundred salamanders that live in the area, although the number is difficult to estimate because they live underground.

"It is the only known breeding pond in the area. Salamanders living on the other side of the road have to cross to reproduce, and if they don't, the population will start to decline," Cook said.

The 10-inch steel pipes are about 35 feet long, with foot-high plastic fencing that acts as funnels to guide the salamanders in.

On recent rainy nights, Bain and some helpers have gathered at the road, picking up salamanders and putting them near the entrances of the pipes.

Sometimes it takes more than one attempt, but many of the 100 salamanders they found did make their way through the tunnels and back.

Even after they left, infrared cameras recorded salamanders using the tunnels without any help from the researchers.